Photovoltaic device with a zinc magnesium oxide window layer

ABSTRACT

Methods and devices are described for a photovoltaic device and substrate structure. In one embodiment, a photovoltaic device includes a substrate structure and a CdTe absorber layer, the substrate structure including a Zn1-xMgxO window layer and a low conductivity buffer layer. Another embodiment is directed to a process for manufacturing a photovoltaic device including forming a Zn1-xMgxO window layer over a substrate by at least one of sputtering, evaporation deposition, CVD, chemical bath deposition process and vapor transport deposition process. The process including forming a CdTe absorber layer above the Zn1-xMgxO window layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 13/238,457 filed Sep. 21, 2011, which claims priority under 35 U.S.C. § 119(e) to Provisional Application No. 61/385,399 filed on Sep. 22, 2010, which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

Embodiments of the invention relate to semiconductor devices and methods of manufacture, and more particularly to the field of photovoltaic (PV) devices.

BACKGROUND OF THE INVENTION

Photovoltaic devices generally comprise multiple layers of material deposited on a substrate, such as glass. FIG. 1 depicts a typical photovoltaic device. Photovoltaic device 100 may employ a glass substrate 105, a transparent conductive oxide (TCO) layer 110 deposited on substrate 105, a window layer 115 made from an n-type semiconductor material, an absorber layer 120 made from a semiconductor material, and a metal back contact 125. Typical devices use cadmium telluride (CdTe) as absorber layer 120 and include glass substrate 105, tin oxide (SnO₂) or cadmium tin oxide (Cd₂SnO₄) as TCO layer 110, and cadmium sulfide (CdS) as the window layer 115. By way of example, a deposition process for a typical photovoltaic device on substrate 105 may be ordered as TCO layer 110 including a n-type material doped with one of SnO₂ and Cd₂SnO₄, CdS window layer 115, a CdTe absorber layer 120, and metal back contact 125. CdTe absorber layer 120 may be deposited on top of window layer 115.

An exemplary energy band diagram of a typical thin-film photovoltaic device, such as a CdTe device is depicted in FIG. 2. Band gap energy for F-doped SnO₂ as TCO layer is depicted as 205, band gap energy of undoped SnO₂ as a buffer layer is depicted as 210, band gap energy of CdS as the window layer is depicted as 215, and band gap energy of CdTe as an absorber layer is depicted as 220. Typically, the conduction band edge offset of CdS relative to CdTe, Δ, is usually −0.2 eV with an experimental uncertainty of +/−0.1 eV.

As depicted in FIG. 2, Δ is the offset in the conduction band edge Ec between the window layer and absorber. In the case of a CdS/CdTe stack, Δ is about −0.2 eV. Theoretical modeling has shown that a more negative Δ leads to larger loss in Voc and FF due to increased rate at which photo carriers recombine at the window/absorber interface. When Δ is made slightly positive (0 to 0.4 eV), the recombination rate can be minimized, leading to improved Voc and FF.

CdS is the conventional window layer in many types of thin-film photovoltaic devices, including photovoltaic devices employing one of CdTe and Cu(In, Ga)Se₂ as an absorber layer. However, as depicted in FIG. 2, the optical band gap for CdS is only 2.4 eV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a typical photovoltaic device.

FIG. 2 depicts an exemplary energy band diagram of a typical thin-film photovoltaic device.

FIG. 3A depicts a substrate structure according to one embodiment.

FIG. 3B depicts a substrate structure according to another embodiment.

FIG. 4 depicts a substrate structure according to another embodiment.

FIG. 5A depicts a thin-film photovoltaic device according to one embodiment.

FIG. 5B depicts a thin-film photovoltaic device according to another embodiment.

FIG. 6 depicts a thin-film photovoltaic device according to another embodiment.

FIG. 7 depicts an energy band diagram of a thin-film photovoltaic device according to one embodiment.

FIG. 8A depicts a graphical representation of open circuit voltage according to one embodiment.

FIG. 8B depicts a graphical representation of quantum efficiency according to another embodiment.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure is directed to photovoltaic devices and methods of production. In one embodiment, Zn_(1-x)Mg_(x)O is employed for a window layer of substrate structure. FIG. 3 depicts substrate structure 300 according to one embodiment. Substrate structure 300 includes substrate 305, transparent conductive oxide (TCO) layer 310, buffer layer 315 and window layer 320. TCO layer 310 may typically be employed to allow solar radiation to enter a photovoltaic device and may further act as an electrode. TCO layer 310 may include an n-type material doped with one of SnO₂ and Cd₂SnO₄. Window layer 320 may be employed to mitigate the internal loss of photo carriers (e.g., electrons and holes) in the device and may strongly influence device parameters including open circuit voltage (Voc), short circuit current (Isc) and fill factor (FF). In one embodiment, window layer 320 may allow incident light to pass to an absorber material to absorb light. According to one embodiment, to improve overall photo emission efficiency of window layer 320, window layer 320 comprises a Zn_(1-x)Mg_(x)O compound.

In one embodiment, substrate structure 300 may include a glass substrate 305, and TCO layer 310. Buffer layer 315 may be optional. Window layer 320 (e.g., Zn_(1-x)Mg_(x)O layer) may be directly on top of TCO layer 310, the TCO layer including one or more of a F-doped SnO₂, undoped SnO₂, and Cd₂SnO₄. When TCO layer 310 includes an undoped Cd₂SnO₄, the TCO layer has no extrinsic dopant, however the layer may be highly n-type due to oxygen vacancies.

According to another embodiment, substrate structure 300 may be provided for manufacturing photovoltaic devices. As depicted in FIG. 3A, the substrate structure includes substrate 305, TCO layer 310, low conductivity buffer layer 315, and Zn_(1-x)Mg_(x)O window layer 320. The substrate structure of FIG. 3A includes Zn_(1-x)Mg_(x)O window layer 320 onto which other layers of a device can be deposited (e.g., absorber layer, metal back, etc.). In one embodiment, Zn_(1-x)Mg_(x)O window layer 320 may be deposited onto a F—SnO₂ based substrate structure (like TEC10). Similarly, substrate structure 300 can be a cadmium stannate (CdSt) substrate structure. Buffer layer 315 may be used to decrease the likelihood of irregularities occurring during the formation of the semiconductor window layer. Buffer layer 315 may be made from a material less conductive than TCO layer 310, such as undoped tin oxide, zinc tin oxide, cadmium zinc oxide or other transparent conductive oxide or a combination thereof. In certain embodiments, substrate structure 300 may not include a buffer layer as depicted in FIG. 4. When substrate structure 300 includes a low conductivity buffer layer 315, the buffer layer is arranged between the substrate 305 (e.g., glass) and the Zn_(1-x)Mg_(x)O window layer.

In one embodiment the thickness of Zn_(1-x)Mg_(x)O window layer 320 ranges from 2 to 2000 nm. In another embodiment, the composition of x in Zn_(1-x)Mg_(x)O is greater than 0 and less than 1. Window layer 320 may be a more conductive material relative to conventional window layer materials, such as CdS. Additionally, window layer 320 may include a window layer material that allows for greatly reduced fill factor (FF) loss in a blue light deficient environment. A Zn_(1-x)Mg_(x)O window layer may allow for more solar radiation in the blue region (e.g., 400 to 475 nm) that can reach the absorber leading to higher short circuit current (Isc).

In an alternative embodiment, a photovoltaic device, such as substrate structure 300 may include a Zn_(1-x)Mg_(x)O compound material as window layer 320 and one or more of a barrier layer and a CdS window layer, as depicted in FIG. 3B. Barrier layer 355 of substrate structure 350 can be silicon oxide, silicon aluminum oxide, tin oxide, or other suitable material or a combination thereof. CdS window layer 360 may be deposited on Zn_(1-x)Mg_(x)O layer 320, wherein the CdS window relates to a surface for depositing an absorber layer. In one embodiment, a photovoltaic device includes a Zn_(1-x)Mg_(x)O window layer, in addition to a substrate structure (e.g., substrate structure 300). For example, substrate structure 300 may utilize a TCO stack including a substrate 305, TCO layer 310 and one or more additional elements. In another embodiment, substrate structure 300 may include buffer layer 315.

Zn_(1-x)Mg_(x)O may be advantageous over a conventional CdS window layers as Zn_(1-x) Mg_(x)O has a wider band gap relative to a device having a CdS window layer. As such, more solar radiation can reach a CdTe absorber, which leads to higher Isc. Similarly, an improved conduction band edge alignment can be achieved by adjusting composition of Zn_(1-x)Mg_(x)O, which leads to higher Voc. The dopant concentration can be in the range of 10¹⁵ to 10¹⁹ atoms (or ions) of dopant per cm³ of metal oxide. Carrier density of Zn_(1-x)Mg_(x)O can be greater than the carrier density of CdS. As such, a stronger n-p semiconductor heterojunction can be formed increasing the built-in potential of the solar cells and minimizing recombination at the interface. A more conductive window layer can also improve the loss in fill factor in a low light environment (e.g., photoconductivity effect), where the percentage of blue light is reduced greatly as compared to under full sun light.

Referring to FIG. 4, the substrate structure of FIG. 3A is depicted according to another embodiment. Substrate structure 400 includes substrate 405, TCO layer 410, and Zn_(1-x)Mg_(x)O window layer 420. Substrate structure 400 may be manufactured at lower cost in comparison to the substrate structure of FIG. 3A

According to another embodiment, Zn_(1-x)Mg_(x)O may be employed for a window layer of a photovoltaic device. FIGS. 5A-5B depict photovoltaic devices according to one or more embodiments, which may be formed as thin-film photovoltaic devices. Referring first to FIG. 5A, photovoltaic device 500 includes substrate 505, transparent conductive oxide (TCO) layer 510, buffer layer 515, window layer 520, absorber layer 525, and metal back contact 530. Absorber layer 525 may be employed to generate photo carriers upon absorption of solar radiation. Metal back contact 530 may be employed to act as an electrode. Metal back contact 530 may be made of molybdenum, aluminum, copper, or any other high conductive materials. Window layer 520 of photovoltaic device 500 may include a Zn_(1-x)Mg_(x)O compound.

More specifically, photovoltaic device 500 may include one or more of glass substrate 505, TCO layer 510 made from SnO₂ or Cd₂SnO₄, buffer layer 515, a Zn_(1-x)Mg_(x)O Window layer 520, a CdTe absorber 525, and a metal back contact 530. Buffer layer 515 may relate to a low conductivity buffer layer, such as undoped SnO₂. Buffer layer 515 may be used to decrease the likelihood of irregularities occurring during the formation of the semiconductor window layer. Absorber layer 525 may be a CdTe layer. The layer thickness and materials are not limited by the thicknesses depicted in FIGS. 5A-5B. In one embodiment, the device of FIG. 5A may employ the substrate of FIG. 3A. In certain embodiments, photovoltaic device 500 may or may not include a low conductivity buffer layer 515, absorber layer 520 a metal back contact 530.

Photovoltaic device 500 may include one or more of a cadmium telluride (CdTe), copper indium gallium (di)selenide (CIGS), and amorphous silicon (Si) as the absorber layer 525. In one embodiment, a photovoltaic device may be provided that includes a Zn_(1-x)Mg_(x)O window layer 520 between a substrate structure, which may or may not include a low conductivity buffer layer 515, and the absorber layer 525. In certain embodiments, the device may additionally include a CdS window layer in addition to Zn_(1-x)Mg_(x)O window layer 520.

In an alternative embodiment, photovoltaic device 500 may include a Zn_(1-x)Mg_(x)O compound material as window layer 520 and one or more of a barrier layer and a CdS window layer as depicted in FIG. 5B. Barrier layer 555 can be silicon oxide, silicon aluminum oxide, tin oxide, or other suitable material or a combination thereof. CdS window layer 560 may be deposited on MS_(1-x)O_(x) layer 520, wherein CdS window layer 560 provides a surface for depositing an absorber layer.

In certain embodiments, photovoltaic device 500 may not include a buffer layer. FIG. 6 depicts thin-film photovoltaic device 600 which includes glass substrate 605, TCO layer 610 made from SnO₂ or Cd₂SnO₄, a MS_(1-x)O_(x) window layer 615, a CdTe absorber 620, and a metal back contact 625

FIG. 7 depicts the band structure of a photovoltaic device, such as a photovoltaic device that employs a CdTe absorber layer, according to one embodiment. In FIG. 7, Band gap energy depicted for F-doped SnO₂ as a TCO layer is depicted as 705, undoped SnO₂ as a buffer layer is depicted as 710, Zn_(1-x)Mg_(x)O as the window layer depicted as 715, and CdTe as the absorber layer is depicted as 720. As further depicted, the conduction band edge offset of Zn_(1-x)Mg_(x)O relative to CdTe, Δ, can be adjusted to 0-0.4 eV. Another advantage of the photovoltaic device of FIG. 3 may be a wider band gap in comparison to CdS.

Both zinc oxide (ZnO) and magnesium oxide (MgO) are wide band gap oxides. ZnO has a band gap of 3.2 eV and MgO has a band gap of about 7.7 eV. ZnO may further be advantageous as it is highly dopable. The ternary compound Zn_(1-x)Mg_(x)O should have a band gap of at least 3 eV as predicted by simulation, which is much larger than that of CdS, and is thus more transparent to blue light. On the other hand, ZnO has a Δ from −0.6 to −1.0 eV relative to CdTe conduction band edge, while MgO has a positive Δ about 2.7 eV. Therefore, the composition of the ternary compound Zn_(1-x)Mg_(x)O can be tuned to lead to a Δ that is slightly positive, as shown in FIG. 4.

FIGS. 8A-8B depict advantages that may be provided by employing Zn_(1-x)Mg_(x)O in the window layer of a photovoltaic device. Referring first to FIG. 8A, a graphical representation is shown of open circuit voltage for a CdS device 805 and Zn_(1-x)Mg_(x)O device 810.

The improvement in device Voc by replacing a CdS window layer with a Zn_(1-x)Mg_(x)O window layer may include improved open circuit voltage from 810 mV to 826 mV. FIG. 8B depicts a graphical representation of quantum efficiency 855 in the range of 400 to 600 nm. As depicted in FIG. 8B, a CdTe device with Zn_(1-x)Mg_(x)O window layer, depicted as 865, has a higher quantum efficiency relative to a CdS window layer, depicted as 860, from 400-500 nm. The current density of Zn_(1-x)Mg_(x)O may relate to 22.6 mA/cm² relative to 21.8 mA/cm² for CdS. The values of Voc improvements described herein are exemplary, as it may be difficult to measure a certain improvement delta. Source current may improve up to 2 mA/cm², wherein the improvement compared to a CdS devices may depend on the thickness of CdS employed.

In another aspect, a process is provided for manufacturing photovoltaic devices and substrates to include a Zn_(1-x)Mg_(x)O window layer as depicted in FIGS. 2 and 3. Substrate structure 200, containing a Zn_(1-x)Mg_(x)O window layer, may be manufactured by one or more processes, wherein one or more layers of the structure may be manufactured by one or more of sputtering, evaporation deposition, and chemical vapor deposition (CVD). Similarly, the Zn_(1-x)Mg_(x)O window layer of photovoltaic device 300 may be manufactured by one or more the following processes, including sputtering, evaporation deposition, CVD, chemical bath deposition and vapor transport deposition.

In one embodiment, a process for manufacturing a photovoltaic device may include a sputtering process of a Zn_(1-x)Mg_(x)O window layer by one of DC Pulsed sputtering, RF sputtering, AC sputtering, and other manufacturing processes in general. The source materials used for sputtering can be one or more ceramic targets of a Zn_(1-x)Mg_(x)O ternary compound, where x is in the range of 0 to 1. In one embodiment, source materials used for sputtering can be one or more targets of Zn_(1-x)Mg_(x) alloy, where x is in the range of 0 to 1. In another embodiment, source materials used for sputtering can be two or more ceramic targets with one or more made from ZnO and the one or more made from MgO. In another embodiment, source materials used for sputtering can be two or more metal targets with one or more made from Zn and one or more made from Mg. Process gas for sputtering the Zn_(1-x)Mg_(x)O can be a mixture of argon and oxygen using different mixing ratios.

In one embodiment, a Zn_(1-x)Mg_(x)O window layer can be deposited by atmospheric pressure chemical vapor deposition (APCVD) with precursors including but not limited to diethyl zinc, bis(cyclopentadienyl)magnesium with reagent such as H₂O or ozone.

According to another embodiment, the process for manufacturing a photovoltaic device may result in a conduction band offset with respect to an absorber layer. For example, the conduction band offset of a window (Zn_(1-x)Mg_(x)O) layer with respect to the absorber layer can be adjusted between 0 and +0.4 eV by choosing the proper value of x. Further, the conductivity of a Zn_(1-x)Mg_(x)O window layer can be adjusted within a range of 1 mOhm per cm to 10 Ohm per cm by doping the zinc magnesium oxide with one of aluminum (Al), manganese (Mn), niobium (Nb), nitrogen (N), fluorine (F), and by introduction of oxygen vacancies. In one embodiment the dopant concentration is from about 1×10¹⁴ cm³ to about 1×10¹⁹ cm³. In one embodiment, the window layer is formed using a sputter target having a dopant concentration from about 1×10¹⁷ cm³ to about 1×10¹⁸ cm³. 

What is claimed is:
 1. A photovoltaic device comprising: a transparent glass substrate; a transparent conductive oxide layer; a Zn_(1-x)Mg_(x)O semiconductor window layer, wherein 0<x<1 and the transparent conductive oxide layer is between the transparent glass substrate and the Zn_(1-x)Mg_(x)O semiconductor window layer; a buffer layer between the transparent conductive oxide layer and the Zn_(1-x)Mg_(x)O semiconductor window layer; and a semiconductor absorber layer on the Zn_(1-x)Mg_(x)O semiconductor window layer, wherein the semiconductor absorber layer comprises cadmium and tellurium, and wherein x is tuned such that the conduction band offset of the Zn_(1-x)Mg_(x)O semiconductor window layer with respect to the semiconductor absorber layer that comprises cadmium and tellurium is in the range of 0 to +0.4 eV; wherein the Zn_(1-x)Mg_(x)O semiconductor window layer is doped with Mn, Nb, N, F or by introducing oxygen vacancies.
 2. The photovoltaic device of claim 1, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer is on the buffer layer.
 3. The photovoltaic device of claim 1, wherein the thickness of the Zn_(1-x)Mg_(x)O semiconductor window layer ranges from about 2 nm to about 2000 nm.
 4. The photovoltaic device of claim 1, wherein the conductivity of the Zn_(1-x)Mg_(x)O semiconductor window layer is within a range of about 1 mOhm per cm to about 10 Ohm per cm.
 5. The photovoltaic device of claim 1, further comprising a barrier layer between the glass substrate and the transparent conductive oxide layer.
 6. The photovoltaic device of claim 1, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer has a dopant concentration of between about 1×10¹⁴ cm⁻³ and about 1×10¹⁹ cm⁻³.
 7. The photovoltaic device of claim 1, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer has a dopant concentration of between about 1×10¹⁷ cm⁻³ and about 1×10¹⁸ cm⁻³.
 8. A method of forming a photovoltaic device, the method comprising: forming a transparent conductive oxide layer over a transparent glass substrate; forming a Zn_(1-x)M_(x)O semiconductor window layer over the transparent conduct oxide layer, wherein 0<x<1 and the transparent conductive oxide layer is between the transparent glass substrate and the Zn_(1-x)Mg_(x)O semiconductor window layer; forming a buffer layer between the transparent conductive oxide layer and the Zn_(1-x)Mg_(x)O semiconductor window layer; and forming a semiconductor absorber layer over the Zn_(1-x)Mg_(x)O semiconductor window layer, wherein the semiconductor absorber layer comprises cadmium and tellurium, and wherein x is tuned such that the conduction band offset of the Zn_(1-x) Mg_(x)O semiconductor window layer with respect to the semiconductor absorber layer that comprises cadmium and tellurium is in the range of 0 to +0.4 eV; and doping the Zn_(1-x)Mg_(x)O semiconductor window layer with Mn, Nb, N, F or by introducing oxygen vacancies.
 9. The method of claim 8 further comprising forming a second semiconductor window layer between the Zn_(1-x)Mg_(x)O semiconductor window layer and the semiconductor absorber layer, wherein the second semiconductor window layer comprises cadmium sulfide.
 10. The method of claim 8, wherein the semiconductor absorber layer is on the Zn_(1-x)Mg_(x)O semiconductor window layer.
 11. The method of claim 8, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer has a dopant concentration of between about 1×10¹⁴ cm⁻³ and about 1×10¹⁹ cm⁻³.
 12. The method of claim 8, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer has a dopant concentration of between about 1×10¹⁷ cm⁻³ and about 1×10¹⁸ cm⁻³.
 13. The method of claim 8, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer is formed by at least one of sputtering, evaporation deposition, CVD, chemical bath deposition process and vapor transport deposition process.
 14. The method of claim 8, wherein the Zn_(1-x)Mg_(x)O semiconductor window layer is formed such that its conductivity is within a range of about 1 mOhm per cm to about 10 Ohm per cm. 